fix: Title underline too short (#1698)

We should add as many "=" as there are letters. The warning breaks CI.

Author: Hyzual

languages/en/deployment-guide/16.x.rst

@@ -2,7 +2,7 @@ Tuleap 16.x
 ###########
 
 Tuleap 16.12 - September 2025
-==========================
+=============================
 
 .. NOTE::